The present invention relates in general to power supply circuits, such as those employed in telecommunication circuit applications, and is particularly directed to a feedback control mechanism for a diode-ORed, redundant power supply system, that effectively xe2x80x98forcesxe2x80x99 the non-dominant power supply to continuously share a portion of the load current, and thereby enables the non-dominant power supply to immediately respond to a load change, such as an interruption in the operation of the dominant power supply.
Designers of power supplies for electronic equipment currently face two significant problems: decreasing voltage requirements and redundancy. The first is due to the fact that, as electronic equipment has become more complex, the industry has been steadily migrating to lower operating voltages in order to reduce power consumption. The second is based upon the need to make this complex equipment more robust than its predecessors. Robustness comes in the form of redundant systems, such as back-up power supplies.
More particularly, back-up power supplies must be designed so that, in the event of a primary supply failure, the back-up or auxiliary power supply will immediately take over without causing an interruption in operation of the circuit being powered. This is especially true in the communications industry, whose products, such as subscriber line interface circuits or xe2x80x98SLICxe2x80x99s, are required to conform with a very demanding performance specification, including accuracy, insensitivity to common mode signals, linearity, low noise, filtering, ease of impedance matching programmability, and low power consumption.
Moreover, as designers of integrated circuits employed for digital communications, such as codecs and the like, continue to xe2x80x98lower the voltage supply rail barxe2x80x99 requirements for their devices (e.g., from five volts down to three volts and below), not only is the communication service provider faced with the problem that such low voltage restrictions may not provide sufficient voltage headroom to provide a low impedance-interface with its existing SLICs (which may be designed to operate at a VCC supply rail of five volts), but the power supply employed must conform with very exacting requirements.
The trend of decreasing output voltages has thus increased the design difficulty. A five volt power supply that is required to maintain a five volt output within a five percent tolerance means that it can swing 250 mV in either direction and still xe2x80x98meet specxe2x80x99. In contrast, a two volt power supply that is required to maintain the same five percent tolerance is allowed to vary only 100 mV in either direction. Obviously, this problem becomes exacerbated as circumstances call for even tighter tolerances and lower voltages.
In the case of redundant power supplies, two or more power supplies are connected in parallel, for example, by means of a relatively common technique called diode-O-Ring, diagrammatically illustrated in FIG. 1, so that if one power supply fails, the other supply will take up the load without causing system shutdown. To this end, the redundant supply of FIG. 1 has a first power supply (P.S.) 1, that produces an output voltage V1, while a second power supply 2 produces an output voltage V2. The two power supplies are connected to an output bus 3 through associated ORing-diodes 4 and 5, respectively, through which currents I1 and I2 flow to a node 6xe2x80x94the point from which an output voltage V3 to a load is supplied.
Typically, the output voltage will vary somewhat from supply to supply, with component tolerances, so that in a redundant arrangement such as shown in FIG. 1, that power supply having the higher reference voltage VREF will supply most of the load current IL. Since the voltage drop across a diode is directly proportional to the current flowing through it, then whichever power supply is sourcing the most current will produce the larger diode drop. On the other hand, the power supply having the lower reference voltage will have a lower output voltage, and supply less current, producing a lower diode voltage drop. With the two power supplies diode-ORed together, equilibrium is quickly reached and each power supply provides some percentage of the currentxe2x80x94provided that the voltage reference differential between the two supplies can be accommodated by the diode voltage drop.
A power supply""s output voltage should not have a large dependence on its output current load. However in the diode-ORing case, the output voltage V3 is dependent on the load or output current IL. In addition, since the voltage drop across the ORing-diode is also dependent on the output current, the output voltage V3 can be hard to predict and regulate. To accommodate this problem, power supplies generally employ some form of closed-loop control that senses the output voltage. This information is fed back to the power supply""s controller and compared to an internal voltage reference, so that the controller may regulate the supply""s output voltage at a constant voltage under different conditions.
In the power supply architecture of FIG. 1, voltage feedback (F.B.) is supplied to reference inputs REF 1, REF 2 from the upstream (V1 and V2) sides of the ORing-diodes 4 and 5, so that the power supplys"" controllers have no information about diode voltage drops. As a result, the output node voltage V3 is subject to the unpredictable voltage drops across the diodes. It is preferred to feed back the voltage from the downstream or V3 side of the ORing-diodes, as shown in FIG. 2, since this is the actual voltage that is being supplied to the load. With the voltage feedback point at the V3 side of the ORing-diodes, the voltage drops across the diodes are no longer a problem, and the power supply with the larger reference voltage will establish the output voltage V3.
On the other hand, the power supply with the lesser reference voltage will turn off, since its feedback exceeds its reference, so that all of the voltage to the load is sourced from the power supply having the higher reference voltage. Therefore, if the active (higher reference voltage) power supply fails suddenly, the inactive supply will be unable to start up immediately, resulting in an interruption in supply voltage to the output node, and likely disruption in the operation of downstream electronic circuitry.
In accordance with the present invention, shortcomings of conventional diode-ORed, redundant power supply systems, such as those described above, are effectively obviated by a new and improved feedback control mechanism that forces the non-dominant or xe2x80x98backupxe2x80x99 power supply to share a (relatively small) portion of the load, which is effective to eliminate the shut off problem when regulating from the load side of the ORing diodes. The regulation control mechanism employed by the invention involves two functional components. The first is to allow regulation on the common output bus of both power supplies. By regulating the voltage on the common output bus, the tolerance error added by the ORing diode is eliminated. The second is to force the non-dominant power supply to share a portion of the load current.
For this purpose, a voltage monitor circuit is coupled across each output-ORing diode. Each monitor circuit measures the voltage drop (Vd) across its associated diode and compares the measured voltage to a prescribed minimum or threshold value. The result of the comparison is then fed back via a summing circuit to the control port of the power supply. If the diode voltage drop Vd as measured by a respective monitor circuit is larger than the minimum value, it is inferred that its power supply is supplying an acceptable portion of the load current to the output node, and the monitor circuit produces a first output value (e.g., a logical high) at its output port.
In response to this output from the monitor circuit, the power supply controller continues to regulate the supply""s output voltage at its present value and the monitor circuit is dormant. However, if the value of the monitored diode voltage Vd drops below its preset minimum value, it is inferred that the other power supply is supplying the entirety of the load current. This causes the output of the monitor circuit to change state, which causes the power supply""s controller to increase the output voltage and forward bias the monitored diode. This effectively xe2x80x98forcesxe2x80x99 the non-dominant power supply to share a portion of the load current, and thereby continuously maintains the non-dominant power supply in a ready-to-go state, so that it can immediately respond to a load change. The monitor circuit also compares the power supply voltage with a further reference voltage, to insure that the power supply output voltage will not rise above a desired maximum.